Birth defects are reported in approximately 3% of all human births and are the largest cause of infant mortality in the United States (Hoyert et al., 2006, Pediatrics; 117:168-183). Exposure to toxic chemicals and physical agents is believed to be responsible for approximately 3% of all birth defects (National Research Council, 2000, “Scientific frontiers in developmental toxicology and risk assessment,” Washington, D.C.: The National Academies Press).
It is understood that developmental toxicity can cause birth defects, and can generate embryonic lethality, intrauterine growth restriction (IUGR), dysmorphogenesis (such as skeletal malformations), and functional toxicity, which can lead to cognitive disorders such as autism. There is an increasing concern about the role that chemical exposure can play in the onset of these disorders. Indeed, it is estimated that 5% to 10% of all birth defects are caused by in utero exposure to known teratogenic agents that induce developmental abnormalities in the fetus (Beckman and Brent, 1984, Annu Rev Pharmacol; 24: 483-500). Concern exists that chemical exposure may be playing a significant and preventable role in producing birth defects (Claudio et al., 2001, Environm Health Perspect; 109: A254-A261).
However, this concern has been difficult to evaluate, due to the lack of robust and efficient models for testing developmental toxicity for the more than 80,000 chemicals in the market, plus the new 2,000 compounds introduced annually (General Accounting Office (GAO), 1994, Toxic Substances Control Act: Preliminary Observations on Legislative Changes to Make TSCA More Effective, Testimony, Jul. 13, 1994, GAO/T-RCED-94-263). Fewer than 5% of these compounds have been tested for reproductive outcomes and even fewer for developmental toxicity (Environmental Protective Agency (EPA), 1998, Chemical Hazard Data Availability Study, Office of Pollution Prevention and Toxins). Although some attempts have been made to use animal model systems to assess toxicity (Piersma, 2004, Toxicology Letters; 149:147-53), inherent differences in the sensitivity of humans in utero have limited the predictive usefulness of such models.
Toxicity, particularly developmental toxicity, is also a major obstacle in the progression of compounds through the drug development process. Currently, toxicity testing is conducted on animal models as a means to predict adverse effects of compound exposure, particularly on development and organogenesis in human embryos and fetuses. The most prevalent models that contribute to FDA approval of investigational new drugs are whole animal studies in rabbits and rats (Piersma, 2004, Toxicology Letters; 149: 147-53). In vivo studies rely on administration of compounds to pregnant animals at different stages of pregnancy and embryonic/fetal development (first week of gestation, organogenesis stage and full gestation length). However, these in vivo animal models are limited by a lack of biological correlation between animal and human responses to chemical compounds during development due to differences in biochemical pathways. Species differences are often manifested in trends such as dose sensitivity and pharmacokinetic processing of compounds. According to the reported literature, animal models are approximately 60% efficient in predicting human developmental response to compounds (Greaves et al., 2004, Nat Rev Drug Discov; 3:226-36). Thus, there is a need for human-directed predictive in vitro models.
The thalidomide tragedy in the 1960s emphasized the importance of preclinical developmental toxicity testing, the significant differences among species in their response to potentially teratogenic compounds, and how the developing fetus can be affected by such compounds. Developmental toxicity testing of thalidomide in rodent models did not indicate the compound's teratogenic potential in humans. Over 10,000 children were born with severe birth defects following in utero exposure. Current preclinical models for detecting developmental toxicity have varying degrees of concordance with observed developmental toxicity in humans, with rats and rabbits (the most commonly used species for developmental toxicity testing) having approximately 70-80% concordance to known human teratogens (Daston G P and Knudsen T B, 2010, “Fundamental concepts, current regulatory design and interpretation,” In: Knudsen T B, Daston G P, editors. Comprehensive Toxicology. Vol 12, 2nd ed. New York: Elsevier. p 3-9). These decades-old in vivo animal models require large numbers of animals, kilogram quantities of test compound, and are both time consuming and expensive. Due to the cost and complexity of these models, safety assessments often occur too late in the compound's life cycle for the developer to react to a positive developmental toxicity signal, and can result in the termination of the development of the compound or series. Though these animal models are, and have long been, considered the regulatory gold standard, differences in species response to a compound may lead to missed signals of developmental toxicity and biological misinterpretation. As such, the development of a new generation of tools using human cells for assessment of potential developmental toxicity risk related to chemical exposure is needed. The appropriate tests would also reduce product development time, control costs, and respond proactively to the call to decrease animal use.
Thus, there is a need for a relevant, predictive, accurate, low cost, and rapid human in vitro tests for reliably determining developmental toxicity of pharmaceutical agents and other chemical compounds.